In recent years, structural health monitoring technologies have been increasingly employed to monitor and detect fastener loosening. The loosening of fasteners on structures can be of great concern, especially when those structures operate in harsh environments or when fasteners are in locations that are difficult to monitor. For example, loosening of bolts or other fasteners on the outer surfaces of a space vehicle can result in loss of pressure or hot gases entering the vehicle during re-entry, possibly resulting in catastrophic failure. As another example, bolts within difficult-to-access areas such as the interior of an airplane wing may be difficult to monitor manually, making it desirable to develop methods of remotely monitoring such fasteners.
Accordingly, it is increasingly desirable to develop and improve methods for detecting loosening of fasteners on structures.